Green fluorescent protein (GFP) derived from Aequorea victoria, a jellyfish, has many purposes in biological systems. Recently, various GFP mutants have been produced based on the random mutagenesis and semi-rational mutagenesis, wherein a color is changed, a folding property is improved, luminance is enhanced, or pH sensitivity is modified. Fluorescent proteins such as GFP are fused with other proteins by gene recombinant technique, and monitoring of the expression and transportation of the fusion proteins is carried out.
One of the most commonly used types of GFP mutant is Yellow fluorescent protein (YFP). Among Aequorea-derived GFP mutants, YFP exhibits the fluorescence with the longest wavelength. The values ε and Φ of the majority of YEPs are 60,000 to 100,000 M−1 cm−1 and 0.6 to 0.8, respectively (Tsien, R. Y. (1998). Ann. Rev. Biochem. 67, 509–544). These values are comparable to those of the general fluorescent group (fluorescein, rhodamine, etc.). Accordingly, improvement of the absolute luminance of YFP is nearly approaching its limit.
Moreover, Cyan fluorescent protein (CFP) is another example of GFP mutants. Among such Cyan fluorescent proteins, ECFP (enhanced cyan fluorescent protein) has been known. Furthermore, Red fluorescent protein (RFP) has been isolated from sea anemone (Discoma sp.), and among such red fluorescent proteins, DasRed has been known. Thus, 4 types of fluorescent proteins including green, yellow, cyan and red fluorescent proteins, have been developed one after another, and their spectrum range has been significantly extended.